Pressure (symbol:
P) is the force per unit area applied in a direction perpendicular
to the surface of an object. The SI unit for pressure is the pascal
(Pa), equal to one Newton per square meter (N/m2 or kg·m-1·s-2).
Non-SI measures such as pound per square inch (psi) and bar are
used in parts of the world. The cgs unit of pressure is the bar
(ba), equal to 1 dyn·cm-2.

In US Customary
units for pressure at sea level, it is 14.696 psi.

Pascal
(Pa)

bar

atmosphere
(atm)

torr
(Torr)

pound-force
per square inch (psi)

1
bar

1
N/m^2

10^6
dyn/cm^2

0.98692

750.06

14.5038

Example:
A woman steps on you foot with a flat shoe one day... not too
much pain. The next day she steps on your foot wearing a pointed
high heel shoe... now much pain. Her weight did not change but
the area where the force applied decreased substantially. Since
pressure P is defined as Force/Area the pressure applied to the
area of the foot is much higher.

This description
of pressure helps explain why in the case of pressure cookers
it does not take much weight (or force of a spring) to control
the pressure when a small hole used.

History:

The first
pressure cooker appeared in France in 1679 introduced as Papin’s
Digester, named for its inventor, the French-born physicist Denis
Papin. The cooker heated water to produce very hot steam which
forced the temperature inside the pot as high as 266° F (130°
C), higher than today's kitchen pressure cookers.

The first
commercial pressure cooker debuted in the United States at the
New York World's Fair in 1939, made by National Presto Industries.
It's defined as a saucepan with a locking lid that creates within
intense steam heat to cook foods in a shorter period of time than
a conventional saucepan.The first patent

food. Then
someone came up with the idea of making large (50 gallon) pressure
cookers to be used for cooking by large businesses and institutions
such as hotels, restaurants and schools. The first US pressure
cooker patents were issued in 1902. The term 'pressure cooker'
first appeared in print in 1915 and by 1917 the US Department
of Agriculture determined that pressure canning was the only safe
method of preserving low acid foods and meats without risking
food poisoning

Then in 1939
the company introduced the first saucepan-style pressure cooker
and gave it the trade name 'Presto 'to signify the speed of pressure-cooking.
The 'Presto 'brand soon became synonymous with pressure-cooking
and eventually the company changed its name to 'National Presto
Company's which you probably know today simply as the Presto Company.
In 1945 they introduced the first truly home-use pressure cooker
which, at 4 quarts, was big enough to cook a meal but small enough
to be manageable by the average housewife

Early pressure
cookers did not have sufficient safety mechanisms and the risk
for explosion was high. Today's modern day pressure cookers can
have several pressure release valve mechanisms are are very safe
if used according to manufacturers instructions.

How do
Pressure Cookers Work?

Normally water
will boil at 212F (100C) at sea level Atmospheric pressure at
1,01325 bar, i.e. normal atmospheric pressure on the sea level
at 0°C --'in an open system'. At this temperature liquid water
turns to gas. Applying more heat to an open pot of water will
only increase the rate that liquid water turns to vapor but will
not increase the temperature of the liquid.

If the pressure
is decreased (e.g., at higher elevations) the point that water
boils will be decreased (since it is easier for water molecules
to escape the surface). If we increase the pressure on water it
becomes more difficult for water molecules to escape the surface
and a higher temperature is required for the water to boil.

In a closed
container things change. The water and vapor are in equilibrium
and will maintain the same temperature. With no place to escape
if heat is applied to the closed container the molecules in the
gas state will increase velocity, hence temperature. This will
increase the pressure on the surface of the liquid. And, the temperature
of the water system will then increase.

Gas Laws

The Ideal
gas law is the equation of state of a hypothetical ideal gas.
It is a good approximation to the behavior of many gases under
many conditions, although it has several limitations. Still it
simply shows how at constant volume increasing pressure will increase
the temperature of the gas.

p
is the absolute pressure of the gas;
V is the volume of the gas; n is the amount of substance of
the gas, usually measured in moles;
R is the gas constant
T is the absolute temperature.

The ideal
gas law does not however take into account the behavior of real
fluids -- the nonzero size of molecules and the attraction between
them. For this one must use the van der Waals equation derived
by Johannes Diderik van der Waals in 1873, who received the Nobel
prize in 1910 for "his work on the equation of state for gases
and liquids". These equations predict the behavior of both gas
and liquid states which exist within the pressure cooker.

Van der Waals first form

p
is the pressure of the fluid
v is the volume of the container holding the particles divided
by the total number of particles
k is Boltzmann's constant
T is the absolute temperature
a' is a measure for the attraction between the particles
b' is the average volume excluded from v by a particle

Upon
introduction of the Avogadro constant NA, the number
of moles n, and the total number of particles n NA,
the equation can be put into the second (better known) form
Van der Waals second form

p
is the pressure of the fluid
V is the total volume of the container containing the fluid
a is a measure of the attraction between the particles
b is the volume excluded by a mole of particles n is the number
of moles
R is the universal gas constant,
T is the absolute temperature

Maillard
Reaction and Pressure Cooking

The Maillard
Reaction is what causes food to brown and gives that nice flavor.
While the Maillard reaction can even take place at room temperature
it starts to take off at temperatures above 250F. So there should
be a greater amount of Maillard reaction ocurring during pressure
cooking than normal simmering of stocks and stews. This is clearly
verified when viewing and tasting stocks cooked using a pressure
cooker.

Pressure
Cooker Advantages:

-- Steam
cooks many foods quicker than dry cooking methods.
-- makes excellent sauces after sautéing by dissolving products
of caramelization and mallaird reaction
--works well with foods that require water infusion/braising
methods
--foods that are steamed retain more nutrients and vitamins
and flavor
--higher temperatures more effective at killing bacteria and
viruses
--saves money since works on very low flame

Pressure
Cooker Disadvantages:

-- takes
time to heat liquid and time for pressure to be released so
foods that cook within 15 minutes does not really save time.
-- cooks using water so foods that benefit by high heat (maillard
reaction) will not produce desirable effects
--everything has to be cooked at the same time
--care must be taken when pressure is released
--while some Maillard reaction occurs food still has a steamed
taste unless sufficient browning is done before or after cooking,

--What
foods are best cooked using a pressure cooker?

Braising
tough cuts of meat --
When cooking meat make sure to sear the meat first just as you
would do when braising.

Beans
--Pressure cooking beans is fast and easy, but it's important
to oil when doing so to keeps the beans from foaming up as well
as keeping the skins from popping. Beans can have a tendency to
clock the air vents so be sure to clean properly after cooking
beans.

Artichokes
-- Making artichokes using a pressure cooker is fairly simple,
and they come out perfectly cooked and very delicious.

Stocks
-- According to Heston Blumenthal of the Fat Duck --..."at
the Fat Duck we've just started using a brilliant gadget to make
stocks. It's not exactly cutting-edge - it's the humble pressure
cooker - but it makes stock better and quicker than any other
method I know of..."

Under cooking
and over cooking foods with the pressure cooker.

One of the
problems you will encounter if just starting with a new pressure
cooking is the times used to cook food. It is best therefore to
abide by the times recommended by the manufacturer since pressure
cookers will vary from one to another. A good rule of thumb is
it is better to undercook than overcook since you can always increase
time.